The present invention relates to an air/fuel ratio sensor for detecting an air/fuel ratio from the concentration of remaining oxygen in an exhaust gas from a vehicle engine, a combined air/fuel ratio sensor for detecting the concentrations of exhaust gas components in addition to the air/fuel ratio, and an engine combustion control system using these air/fuel ratio sensors.
An air/fuel ratio sensor has heretofore been known which detects rich to lean air/fuel ratios utilizing an oxygen pump phenomenon of a zirconia solid electrolyte having oxygen ion conductivity and diffusion controlled phenomena of various exhaust gas components. This type of air/fuel ratio sensor needs to apply heat (e.g., about 700xc2x0 C.) to a sensor element by means of a heater to activate a solid electrolyte.
In order to make it possible to operate the sensor in a short time after power-on, there has recently been proposed a technique for bringing a zirconia solid electrolyte, a platinum electrode, a gas diffusion resistance layer (gas diffusion controlled layer), etc., which form an air/fuel ratio detector, into a laminated structure and laminating such a laminated body (air/fuel ratio detector) and a heater in integral form.
For example, one having a combined structure obtained by integrally calcining a zirconia solid electrolyte placed in a green-sheet state together with a ceramic substrate having a heater built therein has been known in SAE Paper 850379 and the like.
There is also known a combined air/fuel ratio sensor or the like wherein an exhaust gas sensor for detecting the concentrations of HC and NOx in exhaust gas components is integrally combined with an air/fuel ratio detector for detecting a ratio between air and a fuel supplied to an engine.
However, one in which the air/fuel ratio detector of the laminated body is integrally formed with the heater in laminated form, has been not yet in the actual use because of insufficient reliability under its use environment. This is because according to the discussions of the present inventors, a crack is produced in the detector so that the air/fuel ratio sensor is brought into an unserviceable state due to the following reasons. The cause of the occurrence of the crack is roughly divided into the following three groups.
(a) Since the air/fuel ratio detector has a hollow chamber such as an air chamber, it is not possible to effectively transfer the generated amount of heat or heating values of the heater to the detector. Thus, a large temperature gradient occurs between the heater and the air/fuel ratio detector, and the heater is heated at a temperature much higher than at the air/fuel ratio detector. As a result, the detector is apt to break due to a thermal stress developed by the large temperature gradient produced in the direction of the thickness of the detector for the air/fuel ratio sensor. Further, since large warpage occurs in the laminated body having the hollow chamber upon calcining, the laminated body itself is manufactured in a structure apt to break.
(b) A ceramic substrate (which is formed as an alumina substrate in most cases) having a heater built therein and a zirconia solid electrolyte are different in thermal expansion coefficient from each other, and hence a large thermal stress occurs in the detector, so that the detector is apt to break.
(c) A pattern for the heater is concentratedly placed only just below the detector, and hence a sudden temperature gradient occurs in the longitudinal direction of the laminated body of the air/fuel ratio sensor. As a result, a large thermal stress occurs in an end of the heater, which is located on the side opposite to the air/fuel ratio detector as viewed in the longitudinal direction of the laminated body, and the laminated body or the like is apt to break due to the thermal stress.
Thus, the air/fuel ratio sensor having the structure wherein the air/fuel ratio detector is directly heated by the heater, cannot overcome the problem on the reliability. Hence it has been not yet in the actual use. Therefore, the detector should inevitably be heated indirectly by an indirectly heated heater (corresponding to a heater having a structure wherein the heater and detector are separated from each other and a clearance is defined between the two), and a startup time interval (activation time) required to activate the air/fuel ratio sensor was a long time of several tens of seconds. Since the indirectly heated heater is used, power consumption necessary for heating also increases. Since the conventional type air/fuel ratio sensor is long in startup time in particular, this led to a large bottleneck to clear emission control applied immediately after the starting of the engine,
With the foregoing in view, it is therefore an object of the present invention to provide a high-reliable air/fuel ratio sensor which is operated in a short time (e.g., about 5 seconds or less) after power-on so as to comply with emission control immediately after start-up, and which is capable of measuring lean to rich air/fuel ratio ranges with low power consumption and is excellent in heat resistance and durability (crack control function).
It is another object of the present invention to provide an extremely high-implementable and reliable combined air/fuel ratio sensor capable of allowing the air/fuel ratio sensor to combine with the function of detecting exhaust gas components (e.g., HC and NOx) to thereby implement a combined air/fuel ratio and performing exhaust gas component detection in addition to air/fuel ratio detection by making use of oxygen pump action and gas diffusion controlled action in particular.
It is a further object of the present invention to provide a combustion control system suitable for use in an engine, which is capable of easily clearing emission control applied immediately after the start-up of the engine.
In order to achieve the above objects, the present invention is basically constructed as follows:
(a-1) Namely, the present invention is characterized in that an air/fuel ratio detector is formed of a laminated body obtained by stacking an oxygen reference electrode, a dense solid electrolyte, a negative electrode, a porous solid electrolyte, a positive electrode and a porous protection film on one another, and the air/fuel ratio detector and a ceramic substrate having a heater built therein are joined to each other in laminated form.
(a-2) Another invention proposes an air/fuel ratio sensor wherein at least one of an HC detector and an NOx detector each comprised of an oxide semiconductor is provided side by side over a ceramic substrate with a heater built therein by lamination.
(a-3) A further invention proposes a combined air/fuel ratio sensor wherein the above-described air/fuel ratio detector, an NOx detector obtained by stacking a porous oxide, an NOx detecting electrode and a dense solid electrolyte on one another, and a ceramic substrate having a heater built therein are integrally joined to one another so as to be constructed as one laminated structure.
(a-4) A still further invention proposes a combined air/fuel ratio sensor wherein the above-described air/fuel ratio sensor, an HC detector obtained by stacking a porous oxide, an HC detecting electrode and a dense solid electrolyte on one another, and a ceramic substrate having a heater incorporated therein are integrally joined to one another so as to be constructed as a single laminated structure.
(a-5) A still further invention proposes a combined air/fuel ratio sensor wherein the above-described air/fuel ratio detector and a ceramic substrate having a heater built therein are bonded to each other in laminated form, a part of the porous solid electrolyte is hollowed or cut out, a dense solid electrolyte surrounded by insulating materials is formed in the cut-out portion, and an electrode is formed over the upper surface of the dense solid electrolyte, whereby an NOx detector is constructed.
According to the above construction, the present invention can be constructed as a structure in which no hollow chamber is provided in an air/fuel ratio detector. Similarly, the hollow chamber can be deleted even from the HC detector and NOx detector combined with the air/fuel ratio sensor. By doing so, the time required to heat the air/fuel ratio sensor can be advanced or reduced and the air/fuel ratio sensor can be operated in a short time after power-on. Further, a temperature gradient produced in the direction of the thickness of the detector for the air/fuel ratio sensor becomes small, so that a thermal stress developed due to the temperature gradient is reduced. Furthermore, since no hollow chamber exists, a warpage-free laminated body can be obtained.
Even in the case of the laminated structure having no hollow chamber, rich to lean air/fuel ratios can be detected with satisfactory accuracy in the present invention. However, how to detect an air/fuel ratio and exhaust gas components (HC and NOx) will be described in detail by embodiments (see views showing operation principles in FIGS. 5, 14, 16 and 18) to be described later.
(b) In a still further invention as well, insert members (thermal stress buffer layers) each having an intermediate thermal expansion coefficient between a ceramic substrate having a heater built therein and a solid electrolyte (e.g., zirconia solid electrolyte) of an air/fuel ratio detector are placed between the two, whereby a thermal stress developed in a laminated body of an air/fuel ratio sensor can be reduced.
In particular, there is also proposed an air/fuel ratio sensor wherein a laminated body comprised of an electrode constituting an air/fuel ratio detector, an exhaust gas diffusion controlled layer, and a solid electrolyte having oxygen ion conductivity, and a ceramic substrate having a heater built therein are integrally joined to one another with a thermal stress buffer layer interposed therebetween, and the thermal stress buffer layer is comprised of thermal stress buffer layers corresponding to two or more layers which are varied in compounding ratio of a mixed material for the solid electrolyte and the ceramic substrate by the mixed material.
According to the above-described construction, the compounding ratio to the thermal stress buffer layers (ceramic material and mixed material of solid electrolyte and the same material) increases in ceramic material for one layer and increases in solid electrolyte and same material for another layer. Further, the former layer is joined to the ceramic substrate and the latter layer is joined to the solid electrolyte of the air/fuel ratio detector, thereby making it possible to allow each thermal stress buffer layer to approach the air/fuel ratio detector and ceramic substrate. This is effective in preventing cracks.
(c) A still further invention proposes an air/fuel ratio sensor wherein a pattern for the heater is shaped in the form of such a pattern that the air/fuel ratio detector side is dense and gradually becomes coarse as the same faces an end on the opposite side, of the air/fuel ratio detector, as viewed in the longitudinal direction of the laminated body. If done in this way, then a temperature gradient produced in the longitudinal direction of the laminated body of the air/fuel ratio sensor can be also made slow, and hence a thermal stress developed in a heater end directed toward the pad side on the side opposite to the air/fuel ratio detector can be also reduced.
Typical ones of various inventions of the present inventions have been shown in brief. However, the various inventions of the present application and specific configurations of these inventions will be understood from the following description.